1. Field of the Invention
The invention relates to a grinding tool and screw installation plate for use in an improved system for remotely repairing or reconstituting nuclear fuel rod assemblies in the spent fuel pool of a nuclear power plant. The grinding tool and screw installation plate expedites and removal and installation of the bottom nozzles of the fuel assemblies.
2. Description of the Prior Art
Tooling systems for repairing the fuel rod assemblies used in nuclear power plants are known in the prior art. Such tools may be used to either reconstitute a fuel rod assembly having damaged fuel rods, or to reassemble a fuel assembly having a damaged support skeleton. In reconstitution-type repairs, only the defective fuel rods from the undamaged support skeleton are pulled and replaced with new fuel rods. In reassembly-type repairs, all of the undamaged fuel rods are removed from the damaged support skeleton and inserted into a new, undamaged skeleton. In order to implement these two types of repair operations, both manual and computer-operated tooling systems have been developed for individually gripping, lifting, lowering, and ungripping specific fuel rods from one support skeleton to another.
Unfortunately, the use of such tools is not without shortcomings. However, before these shortcomings may be fully appreciated, some brief background as to the structure, operation and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise between two hundred and two-hundred-and-ninety fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from bottom and top nozzles which are interconnected to one another by twenty-four uniformly arrayed thimble tubes. The bottom and top nozzles are eight to nine inches square, and the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 1). The fuel rods themselves are about twelve feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes approximately seven grids, each of which has a square array of open cells for receiving and spacing the fuel rods. The grids are usually formed from flat plates of interlocking sheet metal in an "egg crate" configuration which lends compressive strength to the grids with a minimum of weight. In operation, an array of fuel rod assemblies is lowered into the reactor core by a crane, the control rods of the fuel assemblies are removed, and a jet of pressurized water is guided through the bottom nozzles thereof in order to uniformly absorb the heat generated by the rods. Typically, the velocity of the pressurized water forced through the bottom nozzles of the fuel structures is on the order of fifteen feet per second.
In some nuclear cores, this fifteen feet per second flow of water has created pressure differentials which in turn have resulted in side currents that flow laterally through the fuel rod assemblies disposed in the core. These side currents sometimes produce vibrations in the fuel rods through a fretting action. Additionally, the support skeletons that hold the fuel rods can become damages as a result of routine handling of the fuel assemblies.
In order to repair such damaged fuel rod assemblies, the damaged assembly is typically lowered into the cask-loading area (or shaft) of the spent fuel pool of the nuclear plant. The cask-loading shaft is approximately forty feet long, and filled with water in order to shield workers (who typically stand on a deck located over the shaft) from radiation.
Next, either the top or the bottom nozzle of the damaged assembly is removed in order to afford access to the fuel rods contained therein. In many fuel rod assemblies, the top nozzles are welded onto the thimble tubes of the support skeleton, while the bottom nozzles are attached by a pattern of screws that extend through a pattern of screw holes that are registrable with the bottom ends of the thimble tubes. In such fuel assemblies, the bottom nozzle would be the easiest of the two nozzles to remove. However, in many such fuel assemblies, the feet at the bottom of the nozzles include a flange or gusset-type structure that mechanically interferes with any straightforward access to the thimble screw located below. While it is possible to engage and unscrew the screws located under these flanges by means of a tool having an off-set shaft, the use of such tools has proven in practice to be slow and cumbersome. This is undesirable, since the amount of potentially hazardous radiation that the system operators receive from the spent fuel pool is proportional to the amount of time they must spend over the fuel assemblies disposed in such pools. Still another problem arises when the system operators re-install a bottom nozzle over the fuel assembly after the repair or reconstitution operation has been carried out. To re-install such a bottom nozzle, approximately twenty-four thimble screws must be aligned with the holes in the bottom plate of the bottom nozzle, and screwed into their respective thimble rods. This represents still another time-consuming task, as such installation is typically implemented by long-handled tools at a distance of between about ten and twenty feet.
Clearly, there is a need for a tooling system capable of reliably and expeditiously removing the screw-obstructing portions of the feet of the bottom nozzles of fuel assemblies in order to facilitate the removal of the bottom nozzles. Ideally, the components of such a tooling system should be adjustable to accommodate the different sizes of fuel assemblies. Finally, it would be desirable if such a tooling system included a means for expediting the reinstallation of the bottom nozzle back onto the fuel assembly so as to minimize the amount of radiation exposure that the system operators receive.